Resistor plugs and related issues

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mikenixon
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Resistor plugs and related issues

Post #1 by mikenixon » Tue Feb 05, 2019 6:04 am

Most powersports vehicles come with resistor spark plugs, resistive spark plug wire, or resistor spark plug caps. Many have more than one of these things. Just what are these resistive ignition components for?

In the earliest examples, resistive ignition secondary components are to protect radio communications. The spark plug's high-voltage spark is unfortunately also high frequency that coincides with radio waves and thus can interfere with emergency radio transmissions. By adding a carefully-chosen resistor to the ignition system’s secondary, manufacturers damp the spark’s electrical noise, reducing radio frequency interference (RFI) with only a very small effect on plug voltage. Resistive plug caps by themselves were first. Then resistive plugs were added. Honda issued a bulletin in 1980 announcing the addition of resistive plugs to the already existing resistive caps on their North America-bound street bikes, in response to Canada's then-new laws, which led the US and other countries by decades.

There are other reasons for resistive ignition secondary components as well. On some offroad vehicles, resistive secondary components reduce the electrical backlash that spark creates and which is harmful to the ignition control modules (CDI boxes, igniters).

Resistive technology is also used to shield ignition control modules not connectively but proximitively. Ignition modules that are close enough to the ignition coil are subject to its strong radio waves. It's mostly CDI ignitions that need this protection, plus any vehicle that has an onboard computer, which is virtually all today. Many late model personal watercraft and sport bikes for example won’t even start if the plugs are not resistor type.

However, most vintage streetbikes have resistor plugs or caps or both only as a concession to the increasingly worldwide concern for law enforcement and other official radio transmissions. They do not have electronics onboard that are sensitive to either RFI or backfeed, and there are of course no computers on these vintage machines. It is not unusual therefore for restorers and modifiers of these older machines to convert their ignition systems to non-resistor specification. The main benefit is one less thing to go wrong, to maintain. Additionally however, removing the secondary resistance can increase ignition coil output, offering the ability to use a larger spark plug gap, which can result in two benefits. One, it can increase combustion efficiency to the point that throttle response is sharpened. Two, it can compensate for poor carburetion. But beware of thinking of all higher-voltage ignitions as necessarily improving engine performance. That's just not the way it is. And note: 1975~1979 Gold Wings are unique in that their ignition coils are already high performance type from the factory. You're wasting your money putting Dyna coils on these bikes. The stock coils are so good we used to (and I still do) put them on SOHC inline fours as performance upgrades.

Beware of thinking automotive when servicing or modifying one of these older bikes. Vintage Japanese motorcycle manufacturers have always used plain stranded steel plug wire. Car makers on the other hand generally use resistive plug wire, wire which is not really wire at all, but a mixture of carbon and fibreglas, which due to these materials is very subject to deterioration and very difficult to make good electrical connections with. You don’t ever need to replace the spark plug wires on Asian bikes unless they are broken or badly corroded. You should however unscrew the spark plug caps occasionally and resistance-test the caps, and occasionally trim a little of the spark plug wire off the end so that the cap has fresh surface to bite on when reinstalled. Make it part of regular maintenance. There are several suppliers of bulk spark plug wire. You can also go to an auto parts store and ask for Packard 440 plug wire, 7mm, copper, used on cars in the 1950s. It comes on a 100 ft. roll, Delco part number 1851208.

Watch out for spark plug wire that is embedded in the ignition coils. The really old (pre-1979/1980) points (Kettering) Honda ignition systems had ignition coils with the plug wires molded in place. The wire is not conventionally removable. It is however possible to repair a bad wire on one of these old style coils, but to do so the wire must be extracted from the plastic ignition coil with a pick and the new one epoxied in. There are of course spark plug wire splice kits available, but they're cheesy and make a really poor repair that will likely corrode and get loose. It's really best to just replace the coil.

To summarize, there is more than one reason powersports vehicles have secondary resistance in their ignition systems, and the reasons became more complex and more critical with newer models. Older Japanese four-stroke ignition systems however are subject to fewer maintenance concerns and enjoy a wider choice of options, including modifications to improve serviceability and performance. Finally, the common misconception that bike spark plug wires are like car wires that need to be replaced often has led to unnecessary and unfortunate replacement expense, as well as the application of car-specific resistive wire that has no business on a motorcycle.

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Re: Resistor plugs and related issues

Post #2 by pidjones » Tue Feb 05, 2019 8:13 am

Very good write-up. I bought copper-core 7mm wire from my local O'Rielly store. They had to order a roll. When it came in (next day), they cut off the 7' that I wanted and sent the roll back. I used this to replace the wires on a '79 CB750F which has screw-on connections in the coil similar to the connector in NGK caps. Before buying new caps, I had to pull the caps on the 750 apart and clean the resistor contacts. The elements themselves were still good. Luckily, my GL1000 caps are all still good. BTW, I believe ~5k ohms is the common cap resistance value. From my work with high voltage accelerators, I learned that high voltage resistors develop micro cracks in their resistive films or cores, which are open at low voltage but arc over at high voltage. This can result in very unstable operation, particularly in transient systems like our ignitions.

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Re: Resistor plugs and related issues

Post #3 by Sagebrush » Tue Feb 05, 2019 9:35 am

If you have a Tractor Supply store in your area they carry 4 cylinder solid copper wire plug kits for tractors. There is usually enough wire in one kit to do two early Goldwings.

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Re: Resistor plugs and related issues

Post #4 by mikenixon » Tue Feb 05, 2019 6:32 pm

pidjones wrote:Very good write-up. I bought copper-core 7mm wire from my local O'Rielly store. They had to order a roll. When it came in (next day), they cut off the 7' that I wanted and sent the roll back. I used this to replace the wires on a '79 CB750F which has screw-on connections in the coil similar to the connector in NGK caps. Before buying new caps, I had to pull the caps on the 750 apart and clean the resistor contacts. The elements themselves were still good. Luckily, my GL1000 caps are all still good. BTW, I believe ~5k ohms is the common cap resistance value. From my work with high voltage accelerators, I learned that high voltage resistors develop micro cracks in their resistive films or cores, which are open at low voltage but arc over at high voltage. This can result in very unstable operation, particularly in transient systems like our ignitions.


Yup, Hondas up to mid 80s are 5K new and 10K is allowed. Later caps have higher new and service specs.

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mikenixon
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Re: Resistor plugs and related issues

Post #5 by mikenixon » Tue Feb 05, 2019 6:32 pm

Sagebrush wrote:If you have a Tractor Supply store in your area they carry 4 cylinder solid copper wire plug kits for tractors. There is usually enough wire in one kit to do two early Goldwings.


Cool. Another plus for Tractor Supply. :)

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Re: Resistor plugs and related issues

Post #6 by rcmatt007 » Fri Feb 08, 2019 1:04 pm

as always, I learn something new and what I knew makes better sense.

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Re: Resistor plugs and related issues

Post #7 by Sidecar Bob » Tue Feb 12, 2019 12:18 pm

mikenixon wrote:However, most vintage streetbikes have resistor plugs or caps or both only as a concession to the increasingly worldwide concern for law enforcement and other official radio transmissions. They do not have electronics onboard that are sensitive to either RFI or backfeed, and there are of course no computers on these vintage machines.

Please note that some stereos are very sensitive to RFI from ignitions without resistance in the high tension side. Removing the resistances is common advice in CX/GL500/650 circles and I did that on my GL500 but even with a noise suppressor in the power feed to the stereo AM radio was a complete write off and FM, well the noise wasn't as loud as the exhaust....

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mikenixon
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Re: Resistor plugs and related issues

Post #8 by mikenixon » Tue Feb 12, 2019 8:29 pm

Sidecar Bob wrote:
mikenixon wrote:However, most vintage streetbikes have resistor plugs or caps or both only as a concession to the increasingly worldwide concern for law enforcement and other official radio transmissions. They do not have electronics onboard that are sensitive to either RFI or backfeed, and there are of course no computers on these vintage machines.

Please note that some stereos are very sensitive to RFI from ignitions without resistance in the high tension side. Removing the resistances is common advice in CX/GL500/650 circles and I did that on my GL500 but even with a noise suppressor in the power feed to the stereo AM radio was a complete write off and FM, well the noise wasn't as loud as the exhaust....


Very good point, Sidecar Bob! :)


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